Mailbox body and truck: what they have in common

1. Step 1. Mailbox body — is part of — Letter box. A steel box.
2. Step 2. Letter box — uses — Postal service. The post.
3. Step 3. Postal service — uses — Steam locomotive. Mail trains.
4. Step 4. Steam locomotive — evolved into — Diesel locomotive. It replaced the steam locomotive.
5. Step 5. Diesel locomotive — has part — Air brake. Air brakes.
6. Step 6. Air brake — is part of — Truck. Air brakes, as on trains.

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